A cellular system is conventionally known as a mobile communication system. In addition, a standardization of high-speed wireless communication such as WiMAX (Worldwide Interoperability for Microwave Access), for example, has been recently promoted.
The cellular system prevents radio waves from neighboring wireless base stations from interfering with one another by use of different frequency bands for the neighboring wireless base stations. The cellular system thus establishes a wireless communication network by use of a plurality of different frequency bands.
In contrast, WiMAX adopts TDD (Time Division Duplexing) which transmits and receives radio waves on the same frequency by dividing time for uplink and downlink. By synchronization of frame structures for each wireless base station, WiMAX prevents radio waves transmitted from one terminal from acting as interference radio waves to radio waves transmitted from another wireless base station with which another terminal is communicating.
However, it is assumed that, in light of an effective usage of limited frequencies, unlike the cellular system, it is difficult to assign different frequency bands to the neighboring wireless base stations in a wireless communication system such as WiMAX which uses wide frequency band. For that reason, using the same frequency band at neighboring wireless base stations generates a communication interference region therebetween because of the synchronization of frames between the wireless base stations.
When the communication interference region is generated, since a terminal in the region simultaneously receives radio waves transmitted from neighboring wireless base stations, there is a concern about difficulty to separate the radio waves transmitted from one another in a region where reception intensities of the transmitted radio waves are at approximately the same level, which leads to a failure of handover to the neighboring wireless base station and disconnection.
Meanwhile, in order to reduce a dead region caused by interference, Japanese Patent Application Laid-open No. 11-308037, for example, suggests to form a null in directions of a plurality of wireless base stations by adopting adaptive arrays to antennas of the plurality of wireless base stations.